1. Field of the Invention
This invention relates to an improved process for the removal of toxic cyanide and anionic metal species from alkaline solutions resulting from the mining and plating industries.
2. Description of the Related Art
Gold occurs primarily as the native metal, alloyed with silver or other metals or as tellurides. It is commonly associated with the sulfides of iron, silver, arsenic, antimony and copper. Silver occurs as finely disseminated metal in rocks of hydrothermal origin as silver chloride, sulfide or tellurides and as complex sulfides with antimony and arsenic. Historical practice with ores containing native metal involve crushing, concentration of the gold or silver by gravity separation and recovery by amalgamation with mercury. Environmental concerns have resulted in abandonment of this process in most cases.
Two processes, heap leaching and carbon-in-pulp (CIP), are used for the recovery of gold from ore depending to a large extent on the grade and nature of the ore. Both processes result in waste streams consisting of dilute solutions containing low levels of cyanide, metal cyanide complexes, and depending on the ore, other toxic metal species such as arsenate or selenate. Low grade ores consisting of gold disseminated in largely silaceous rock are typically leached by piling the crushed ore on specially prepared pads to a depth of several feet and then continuously distributing an aqueous cyanide solution across the surface of the heap. As the cyanide solution trickles through the ore, the gold is leached from the ore as the soluble aurocyanide species. The gold bearing leach solution is collected at the bottom of the heap and pumped to a treatment facility for recovery of the gold. When the gold tenor in the leach solution drops to the point where it is no longer economically feasible to treat the ore, leaching is stopped and the heap is abandoned. At this point, the heap is saturated with dilute aqueous cyanide solution containing various other metal cyanide complexes as well as potentially other toxic metal species. This solution must be washed from the heap and treated to destroy the various cyanide species and remove the remaining toxic metal species. If the heap is not washed, these cyanide species and toxic metal species will continue to leach from the heap over time, resulting in an environmental threat to wildlife and groundwater.
Higher grade ores or ores in which the gold is locked in a matrix which is not easily leached are treated by CIP processes. The ore is finally ground and placed in a leaching vessel containing alkaline cyanide solution and carbon. The gold is adsorbed by the carbon as it is leached. The spent slurry is then subjected to a series of solid/liquid separation steps before being deposited in a tailings dam as a thickened slurry. Water continues to separate over time from these tailings. This water contains low levels of cyanide and metal cyanide species. It must be treated prior to discharge into the environment or in some cases, before return to the leaching process. Alternatively, the thickened slurry can be treated by contacting with a solid phase adsorbent prior to discharge to the tailings bond to remove the cyanide and anionic metal species.
In both cases, the levels of cyanide species and toxic metals such as arsenic and selenium are very low, 1-10 ppm. In order to achieve essentially complete removal by standard oxidation processes with hydrogen peroxide or hypochlorite and precipitation processes, large excesses of reagents are required. If the toxic species can be concentrated, then treatment can be carried out much more efficiently with considerable savings in reagent costs.
Similarly, spent electroplating solutions and rinse solutions containing heavy metal cyanides such as zinc cyanide or cadmium cyanide are often dilute, requiring large excesses of reagents for their removal. Concentration of these species would also allow savings in reagent costs.